The present invention relates to drilling systems for stabilizing and directing drilling bits and particularly to eccentric adjustable diameter stabilizers for stabilizing and controlling the trajectory of drilling bits and more particularly to bi-center bits.
In the drilling of oil and gas wells, concentric casing strings are installed and cemented in the borehole as drilling progresses to increasing depths. In supporting additional casing strings within the previously run strings, the annular space around the newly installed casing string is limited. Further, as successive smaller diameter casings are suspended within the well, the flow area for the production of oil and gas is reduced. To increase the annular area for the cementing operation and to increase the production flow area, it has become common to drill a larger diameter new borehole below the terminal end of the previously installed casing string and existing cased borehole so as to permit the installation of a larger diameter casing string which could not otherwise have been installed in a smaller borehole. By drilling the new borehole with a larger diameter than the inside diameter of the existing cased borehole, a greater annular area is provided for the cementing operation and the subsequently suspended new casing string may have a larger inner diameter so as to provide a larger flow area for the production of oil and gas.
Various methods have been devised for passing a drilling assembly through the existing cased borehole and permitting the drilling assembly to drill a larger diameter new borehole than the inside diameter of the upper existing cased borehole. One such method is the use of underreamers which are collapsed to pass through the smaller diameter existing cased borehole and then expanded to ream the new borehole and provide a larger diameter for the installation of larger diameter casing. Another method is the use of a winged reamer disposed above a conventional bit.
Another method for drilling a larger diameter borehole includes a drilling assembly using a bi-center bit. Various types of bi-center bits are manufactured by Diamond Products International, Inc. of Houston, Tex. See the Diamond Products International brochure incorporated herein by reference.
The bi-center bit is a combination reamer and pilot bit. The pilot bit is disposed on the downstream end of the drilling assembly with the reamer section disposed upstream of the pilot bit. The pilot bit drills a pilot borehole on center in the desired trajectory of the well path and then the eccentric reamer section follows the pilot bit reaming the pilot borehole to the desired diameter for the new borehole. The diameter of the pilot bit is made as large as possible for stability and still be able to pass through the cased borehole and allow the bi-center bit to drill a borehole that is approximately 15% larger than the diameter of the existing cased borehole. Since the reamer section is eccentric, the reamer section tends to cause the pilot bit to wobble and undesirably deviate off center and therefore from the preferred trajectory of drilling the well path. The bi-center bit tends to be pushed away from the center of the borehole because the resultant force of the radial force acting on the reamer blade caused by weight on bit and of the circumferential force caused by the cutters on the pilot bit, do not act across the center line of the bi-center bit. Because this resultant force is not acting on the center of the bi-center bit, the bi-center bit tends to deviate from the desired trajectory of the well path.
The drilling assembly must have a pass through diameter which will allow it to pass through the existing cased borehole. The reamer section of the bi-center bit is eccentric. It is recommended that the stabilizer be located approximately 30 feet above the reamer section of the bi-center bit to allow it to deflect radially without excessive wedging as it is passes through the upper existing cased borehole. If the eccentric reamer section is located closer to the stabilizer, the drilling assembly would no longer sufficiently deflect and pass through the upper existing cased borehole. The stabilizer and collars must allow the bi-center bit to deflect radially without excessive wedging as it passes through the existing cased borehole.
Typically a fixed blade stabilizer is mounted on the drilling assembly. The fixed blade stabilizer includes a plurality of blades azimuthally spaced around the circumference of the housing of the stabilizer with the outer edges of the blades being concentric and adapted to contact the wall of the existing cased borehole. The stabilizer housing has approximately the same outside diameter as the bi-center bit. Obviously, the fixed blade stabilizer must have a diameter which is smaller than the inside diameter of the upper existing cased borehole, i.e. pass through diameter. In fact the fixed blade stabilizer must have a diameter which is equal to or less than outside diameter of the pilot bit of the bi-center bit. Therefore, it can be appreciated that the blades of the fixed blade stabilizer will not all simultaneously contact the wall of the new borehole since the new borehole will have a larger diameter than that of the upper existing cased borehole. By not all of the fixed blades engaging the wall of the new larger diameter borehole, the fixed blade stabilizer is not centralized within the new borehole and often cannot prevent the resultant force on the bi-center bit from causing the center line of the pilot bit from deviating from the center line of the preferred trajectory of the borehole.
An adjustable concentric blade stabilizer may be used on the drilling assembly. The adjustable stabilizer allows the blades to be collapsed into the stabilizer housing as the drilling assembly passes through the upper existing cased borehole and then expanded within the new larger diameter borehole whereby the stabilizer blades engage the wall of the new borehole to enhance the stabilizer's ability to keep the pilot bit center line in line with the center line of the borehole. As the eccentric reamer on the bi-center bit tends to force the pilot bit off center, the expanded adjustable stabilizer blades contacts the opposite side of the new borehole to counter that force and keep the pilot bit on center.
One type of adjustable concentric stabilizer is manufactured by Halliburton, Houston, Texas and is described in U.S. Pat. Nos. 5,318,137; 5,318,138; and 5,332,048, all incorporated herein by reference. Another type of adjustable concentric stabilizer is manufactured by Anderguage U.S.A., Inc., Spring, Tex. See Andergauge World Oil article and brochure incorporated herein by reference.
Even with adjustable concentric blade stabilizers, it is still recommended that the stabilizer be located at least 30 feet above the bi-center bit. The outside diameter of the housing of an adjustable concentric diameter blade stabilizer is slightly greater than the outside diameter of the steerable motor. The adjustable blade stabilizer housing includes a large number of blades azimuthally spaced around its circumference and extending radially from a central flow passage passing through the center of the stabilizer housing. To fit a large number of blades interiorally of the housing, it is necessary to increase the outer diameter of the housing. This produces an offset on the housing. However, the outside diameter of the adjustable stabilizer housing must not exceed the outside diameter of the pilot bit if the adjustable stabilizer is to be located within 30 feet of the bi-center bit. Even if the outside diameter is only increased 1/2 of an inch, for example, there would not be adequate deflection of the drilling assembly to allow the passage of the drilling assembly down through the existing cased borehole.
The stabilizer is so far away from the bi-center bit that it cannot prevent the eccentric reamer section from tending to push off the wall of the new borehole and cause the pilot bit to deviate from the center line of the trajectory of the well path thereby producing a borehole which is undersized, i.e. produces a diameter which is less than the desired diameter. Such drilling may produce an undersized borehole which is approximately the same diameter as would have been produced by a conventional drill bit.
By locating the stabilizer approximately 30 feet above the bi-center bit, the deflection angle between the stabilizer and the eccentric reamer section is so small that it does not affect the pass through of the drilling assembly. However, as the stabilizer is moved closer to the bi-center bit, the deflection angle becomes greater until the stabilizer is too close to the bi-center bit which causes it to wedge in the borehole and not allow the assembly to pass through the existing cased borehole.
It is preferred that the stabilizer be only two or three feet above the bi-center bit to ensure that the pilot bit drills on center. Having the stabilizer near the bi-center bit is preferred because not only does the stabilizer maintain the pilot bit on center, but the stabilizer also provides a fulcrum for the drilling assembly to direct the drilling direction of the bit. This can be appreciated by an understanding of the various types of drilling assemblies used for drilling in a desired direction whether the direction be a straight borehole or a deviated borehole.
A pendulum drilling assembly includes a fixed blade stabilizer located approximately 30 to 90 feet above the conventional drilling bit with drill collars extending therebetween. The fixed stabilizer acts as the fulcrum or pivot point for the bit. The weight of the drill collars causes the bit to pivot downwardly under the force of gravity on the drill collars to drop hole angle. However, weight is required on the longitudinal axis of the bit in order to drill. The sag of the drill collars below the stabilizer causes the centerline of the drill bit to point above the direction of the borehole being drilled. If the inclination of the borehole is required to decrease at a slower rate, more weight is applied to the bit. The greater resultant force in the upward direction from the increased weight on bit, offsets part of the side force from the drill collar weight causing the borehole to be drilled with less drop tendency. Oftentimes the pendulum assembly is used to drop the direction of the borehole back to vertical. The pendulum assembly's directional tendency is very sensitive to weight on bit. Usually the rate of penetration for drilling the borehole is slowed down dramatically in order to maintain an acceptable near vertical direction.
A packed hole drilling assembly typically includes a conventional drill bit with a lower stabilizer approximately 3 feet above the bit, an intermediate stabilizer approximately 10 feet above the lower stabilizer and then an upper stabilizer approximately 30 feet above the intermediate stabilizer. A fourth stabilizer is not uncommon. Drill collars are disposed between the stabilizers. Each of the stabilizers are full gauge, fixed blade stabilizers providing little or no clearance between the stabilizer blades and the borehole wall. The objective of a packed hole drilling assembly is to provide a short stiff drilling assembly with as little deflection as possible so as to drill a straight borehole. The packed hole assembly's straight hole tendency is normally insensitive to bit weight.
A rotary drilling assembly can include a conventional drilling bit mounted on a lower stabilizer which is typically disposed 21/2 to 3 feet above the bit. A plurality of drill collars extends between the lower stabilizer and other stabilizers in the bottom hole assembly. The second stabilizer typically is about 10 to 15 feet above the lower stabilizer. There could also be additional stabilizers above the second stabilizer. Typically the lower stabilizer is 1/32 inch under gage to as much as 1/4 inch under gage. The additional stabilizers are typically 1/8 to 1/4 inch under gage. The second stabilizer may be either a fixed blade stabilizer or more recently an adjustable blade stabilizer. In operation, the lower stabilizer acts as a fulcrum or pivot point for the bit. The weight of the drill collars on one side of the lower stabilizer can move downwardly, until the second stabilizer touches the bottom side of the borehole, due to gravity causing the longitudinal axis of the bit to pivot upwardly on the other side of the lower stabilizer in a direction so as to build drill angle. A radial change of the blades, either fixed or adjustable, of the second stabilizer can control the vertical pivoting of the bit on the lower stabilizer so as to provide a two dimensional gravity based steerable system so that the drill hole direction can build or drop inclination as desired.
Steerable systems, as distinguished from rotary drilling systems, include a bottom hole drilling assembly having a steerable motor for rotating the bit. Typically, rotary assemblies are used for drilling substantially straight holes or holes which can be drilled using gravity. Gravity can be effectively used in a highly deviated or horizontal borehole to control inclination. However, gravity can not be used to control azimuth. A typical bottom hole steerable assembly includes a bit mounted on the output shaft of a steerable motor. A lower fixed or adjustable blade stabilizer is mounted on the housing of the steerable motor. An adjustable blade stabilizer on the motor housing is not multi-positional and includes either a contracted or expanded position. The steerable motor includes a bend, typically between 3/4.degree. and 3.degree.. Above the steerable motor is an upper fixed or concentrically adjustable blade stabilizer or slick assembly. Typically, the lower fixed blade stabilizer is used as the fulcrum or pivot point whereby the bottom hole assembly can build or drop drilling angle by adjusting the blades of the upper concentrically adjustable stabilizer. The upper concentrically adjustable stabilizer may be multi-positional whereby the stabilizer blades have a plurality of concentric radial positions from the housing of the stabilizer thereby pivoting the bit up or down by means of the fulcrum of the lower fixed blade stabilizer. It is known to mount a concentric adjustable blade stabilizer below the motor on the motor's output shaft between the bit and the motor with the concentric adjustable blade stabilizer rotating with the bit. One of the principal advantages of the steerable motor is that it allows the bit to be moved laterally or change azimuth where a conventional rotary assembly principally allows the bit to build or drop drilling angle.
The steerable drilling assembly includes two drilling modes, a rotary mode and a slide mode. In the rotary drilling mode, not only does the bit rotate by means of the steerable motor but the entire drill string also rotates by means of a rotary table on the rig causing the bend in the steerable motor to orbit about the center line of the bottom hole assembly. Typically the rotary drilling mode is used for drilling straight ahead or slight changes in inclination and is preferred because it offers a high drilling rate.
The other drilling mode is the slide mode where only the bit rotates by means of the steerable motor and the drill string is no longer rotated by the rotary table at the surface. The bend in the steerable motor is pointed in a specific direction and only the bit is rotated by fluid flow through the steerable motor to drill in the preferred direction, typically to correct the direction of drilling. The remainder of the bottom hole assembly then slides down the hole drilled by the bit. The rotation of the bit is caused by the output of the drive shaft of the steerable motor. The slide mode is not preferred because it has a much lower rate of drilling or penetration rate than does the rotary mode.
It can be seen that the rotary assembly and the steerable assembly with a conventional drill bit rely upon a stabilizer to act as a fulcrum or pivot point for altering the direction of drilling of the bit. When a bi-center bit is used with these drilling assemblies, near bit stabilization cannot be achieved because the nearest stabilizer can only be located approximately 30 feet above the bi-center bit because the drilling assembly must pass through the upper existing cased borehole. With the closest stabilizer being 30 feet above the bi-center bit, the drilling assembly becomes a pendulum drilling assembly and, as previously discussed, poses a problem for controlling the center line of the pilot bit and thus the direction of drilling. As with a pendulum assembly, the bit is tilted in a direction to build angle. With a normal pendulum assembly, the gravitational force acts on the bit to cause it to side cut to the low side so that the bit tilt effect may not be predominate, depending on weight on bit, drilling rate, rock properties, bit design, etc. For most bi-center bits, the lateral force from the reamer is greater than the gravity force at low inclinations, thus the bit does not side cut only on the low side, but cuts in all directions around the hole. This causes the bit tilt to predominate and, thus the bi-center bit may build angle more readily than a standard bit. Thus it can be seen that the best possible bottom hole assembly with a bi-center bit has greater instability than a comparable bottom hole assembly with a standard bit. Because of this instability, rotary assemblies with fixed blade stabilizers would require constant changing, tripping in and out of the borehole, to change to a stabilizer with a different diameter for borehole inclination correction. Also, because of this instability, steerable assemblies require a lot of reorienting of the hole direction to correct the direction of drilling, thus requiring the use of the sliding mode of drilling with its low penetration rate.
Also, drilling in the sliding mode often produces an abrupt dog leg or kink in the borehole. Ideally, there should be no abrupt change in direction. Although a gradual consistent dog leg of 2.degree. in 100 feet is not detrimental, and an abrupt change of 2.degree. at one location every 100 feet is detrimental. Abrupt changes in drilling trajectory causes tortuosity. Tortuosity is a term describing a borehole which has the trajectory of a corkscrew which causes the borehole to have many changes in direction forming a very tortuous well path through which the bottom hole assembly and drill string trip in and out of the well. Tortuosity substantially increases the torque and drag on the drill string. In extended reach drilling, tortuosity limits the distance that the drill string can drill and thus limits the length of the extended reach well. Tortuosity also limits the torque that can effectively be placed in the bottom hole assembly and causes the drill string or bottom hole assembly to get stuck in the borehole. The article, entitled "Use of Bicenter PDC Bit Reduces Drilling Cost" by Robert G. Casto in the Nov. 13, 1995 issue of Oil & Gas Journal, describes the deficiencies of drilling in the slide mode. It should be appreciated that rig costs are extraordinarily expensive and therefore it is desirable to limit slide mode drilling as much as possible.
The prior art previously discussed is more directed to lower angle drilling. For high angle drilling, the reamer section of the bi-center bit tends to ream and undercut the bottom side of the hole causing the bit to drop angle. This is very formation dependant and makes the bi-center bit even more unstable and unpredictable.
The present invention overcomes the deficiencies of the prior art.